About 14 per cent of pregnant girls ages 12 to 17 smoke marijuana, compared to roughly 6 per cent of their non-pregnant peers, the study found. By contrast, older pregnant women were at least half as likely to smoke pot as their peers.

Among all pregnant women ages 12 to 44 in the study, about 6 per cent reported marijuana use during the first trimester, while 3 per cent said they smoked pot in the second trimester and about 2 per cent use the drug in the third and final trimester.

“These findings should be considered in a broader context in which young people increasingly view marijuana not just as a benign, but even as a beneficial drug,” said lead study author Dr. Nora Volkow, director of the National Institute on Drug Abuse in Bethesda, Maryland.

“The higher prevalence of marijuana use observed in the first trimester as opposed to later in the pregnancy, suggests that some women are likely to resort to marijuana for managing their nausea,” Volkow added by email.

Doctors advise women to completely avoid marijuana during pregnancy because it may have negative effects on fetal neurodevelopment, especially during the first trimester.

For the study, researchers examined survey data on marijuana use reported by 410,000 women aged 12 to 44, including 14,400 who were pregnant. The national survey data was collected from 2002 to 2015.

Overall, about 4 per cent of pregnant women and almost 8 per cent of non-pregnant women reported using marijuana, researchers report in the Annals of Internal Medicine.

In both pregnant and non-pregnant groups, women who were at least 26 years old were less likely to use marijuana than younger women.

Among pregnant women, black women were more likely to report marijuana use than other racial and ethnic groups.

One limitation of the study is that it relied on women to accurately report their drug use, which may mean the results underestimate how much people use it, the authors note.

A woman exhales while smoking a joint during the annual 420 marijuana rally on Parliament hill on Wednesday, April 20, 2016 in Ottawa.

Justin Tang/THE CANADIAN PRESS

Even so, the findings are a reminder that despite declining teen pregnancy rates, parents and doctors should still keep in mind that these young women may be more likely to engage in risky behaviors than their peers, said Patricia Cavazos-Rehg of the Washington University School of Medicine in St. Louis.

“We have known for some time that certain groups of adolescents are prone to deviant behaviors, but we need to know more about the unique environmental and/or biological vulnerability factors that influence the tendency for risk behavior engagement at the outset,” Cavazos-Rehg, who wasn’t involved in the study, said by email.

The trouble with pot smoking for pregnant teens is it may be even more dangerous for them than for older women, said Dr. Nathaniel DeNicola of George Washington University in Washington, D.C.

“Drug use, and marijuana use specifically, is underreported by all age groups, and there is reason to think this is higher among teens,” DeNicola, who wasn’t involved in the study, said by email.

“But the bigger issue: marijuana use is different among teens – it’s more dangerous,” DeNicola added. “There are more adverse and consequential impacts on the still-developing brain.”

For some teens, just telling about the risk of pot during pregnancy may be enough to get them to stop, noted Dr. Seth Ammerman of Stanford University and Lucile Packard Children’s Hospital in California.

“Some of these adolescents who are using do not initially realize they are pregnant, but once they find out and are counseled to stop use, a significant number do, so rates decrease from the first trimester on,” Ammerman, who wasn’t involved in the study, said by email.